This invention is in the field of plug bodies used in double block plug valves and diverter plug valves found in pipeline applications.
A plug valve includes a tapered (wedge-shaped) central or main plug body that moves the valve's seals (permanently bonded into slips) to and from the valve body (see e.g. FIG. 1). In the open position, the seals are completely out of the product flow. Turning the operator hand wheel clockwise rotates the plug body 90° to block flow. During rotation, clearance is maintained between the slip seals and the inside diameter bore of the valve body. When the operator hand wheel is turned further clockwise, the wedge-shaped plug body begins to lower, forcing the slips and their seals outward against the inside diameter bore of the valve body. This produces a seal that holds the pressure of the medium flowing into and through the valve body.
The valve's operator includes a stem or upper trunnion that, unlike the main plug body, is exposed to the environment. Sealing around the upper trunnion is critical to ensure product does not leak out into the environment. Additionally, the upper trunnion experiences the torque load required to open and close the valve during operation. Therefore, some plug valve customers require the upper trunnion to be made from a different material than the main plug body. For example, the upper trunnion can be specified to be made from a stronger and more corrosion resistant material than the rest of the plug body.
The three main ways to meet this different material requirement are: (1) cast the upper trunnion and main plug body as a one-piece casting using the stronger, more corrosion resistant material specified for the upper trunnion (see e.g. FIG. 2); (2) cast the upper trunnion separately using the stronger, corrosion resistant material and then cast the main plug body around the upper trunnion using a different material; or (3) cast each part separately and attach the upper trunnion to the main plug body with a pin. The one-piece casting option makes the plug more expensive because the main plug body is being cast using the stronger, more corrosion resistant material. The cast-around option is difficult to do and results in a relatively high scrap rate due to casting defects. The pinned option introduces a potential failure mode during valve operation.